skbuff: Extract list pointers to silence compiler warnings

Under both -Warray-bounds and the object_size sanitizer, the compiler is
upset about accessing prev/next of sk_buff when the object it thinks it
is coming from is sk_buff_head. The warning is a false positive due to
the compiler taking a conservative approach, opting to warn at casting
time rather than access time.

However, in support of enabling -Warray-bounds globally (which has
found many real bugs), arrange things for sk_buff so that the compiler
can unambiguously see that there is no intention to access anything
except prev/next. Introduce and cast to a separate struct sk_buff_list,
which contains _only_ the first two fields, silencing the warnings:

In file included from ./include/net/net_namespace.h:39,
from ./include/linux/netdevice.h:37,
from net/core/netpoll.c:17:
net/core/netpoll.c: In function 'refill_skbs':
./include/linux/skbuff.h:2086:9: warning: array subscript 'struct sk_buff[0]' is partly outside array bounds of 'struct sk_buff_head[1]' [-Warray-bounds]
2086 | __skb_insert(newsk, next->prev, next, list);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/core/netpoll.c:49:28: note: while referencing 'skb_pool'
49 | static struct sk_buff_head skb_pool;
| ^~~~~~~~

This change results in no executable instruction differences.

Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/skbuff.h

···
 #endif
 
 struct sk_buff_head {
-	/* These two members must be first. */
-	struct sk_buff	*next;
-	struct sk_buff	*prev;
+	/* These two members must be first to match sk_buff. */
+	struct_group_tagged(sk_buff_list, list,
+		struct sk_buff	*next;
+		struct sk_buff	*prev;
+	);
 
 	__u32		qlen;
 	spinlock_t	lock;
···
 struct sk_buff {
 	union {
 		struct {
-			/* These two members must be first. */
+			/* These two members must be first to match sk_buff_head. */
 			struct sk_buff		*next;
 			struct sk_buff		*prev;
 
···
 	 */
 	WRITE_ONCE(newsk->next, next);
 	WRITE_ONCE(newsk->prev, prev);
-	WRITE_ONCE(next->prev, newsk);
-	WRITE_ONCE(prev->next, newsk);
+	WRITE_ONCE(((struct sk_buff_list *)next)->prev, newsk);
+	WRITE_ONCE(((struct sk_buff_list *)prev)->next, newsk);
 	WRITE_ONCE(list->qlen, list->qlen + 1);
 }
 
···
 				     struct sk_buff *prev,
 				     struct sk_buff *newsk)
 {
-	__skb_insert(newsk, prev, prev->next, list);
+	__skb_insert(newsk, prev, ((struct sk_buff_list *)prev)->next, list);
 }
 
 void skb_append(struct sk_buff *old, struct sk_buff *newsk,
···
 				      struct sk_buff *next,
 				      struct sk_buff *newsk)
 {
-	__skb_insert(newsk, next->prev, next, list);
+	__skb_insert(newsk, ((struct sk_buff_list *)next)->prev, next, list);
 }
 
 /**